The subject of the present invention is a graphite cathode for the electrolysis of aluminium.
In the electrolytic process used in most aluminium production plants, an electrolysis pot comprises, in a metal box jacketed with refractories, a cathode sole composed of several juxtaposed cathode blocks. This assembly constitutes the crucible which, rendered sealed by a fireproof-lining slurry, is the seat of the transformation, under the action of the electric current, from electrolytic solution into aluminium. This reaction takes place at a temperature generally greater than 950xc2x0 C. In order to withstand the thermal and chemical conditions prevailing during the operation of the pot and to satisfy the need to conduct the elecrolysis current, the cathode block is manufactured from carbon-containing materials. These materials range from semi-graphite to graphite. They are formed by extrusion or by vibrocompaction after mixing the raw materials:
either a mixture of pitch, calcined anthracite and/or graphite in the case of semi-graphitic and graphitic materials. These materials are then fired at approximately 1200xc2x0 C. The graphitic cathode contains no anthracite. The cathode manufactured from these materials is commonly called xe2x80x9ccarbon cathodexe2x80x9d.
or a mixture of pitch and coke, with or without graphite in the case of graphites. In this case, the materials are fired at approximately 800xc2x0 C. and then graphitized at above 2400xc2x0 C. This cathode is called a xe2x80x9cgraphite cathodexe2x80x9d.
It is known to use carbon cathodes which, however, have moderate electrical and thermal properties, no longer suitable for the operating conditions in modern pots, especially a high current intensity. The need to reduce energy consumption and the possibility of increasing the intensity of the current, especially in existing potlines, has prompted the use of graphite cathodes.
The graphitization treatment for graphite cathodes, at above 2400xc2x0 C., allows the electrical and thermal conductivities to be increased, thus creating the conditions sufficient for optimized operation of an electrolysis pot. The energy consumption decreases because of the drop in electrical resistance of the cathode. Another way of benefiting from this drop in electrical resistance consists in increasing the intensity of the current injected into the pot, thus making it possible to increase the production of aluminium. The high value of the thermal conductivity of the cathode then allows the excess heat generated by the increased current to be removed. In addition, graphite-cathode pots appear to be electrically less unstable, that is to say having less fluctuation in electric potentials than carbon-cathode pots.
However, it has turned out that pots equipped with graphite cathodes have a shorter lifetime than pots equipped with carbon cathodes. Graphite-cathode pots become unusable by the aluminium being excessively enriched with iron, which results from the cathode bar being corroded by the aluminium. The metal reaches the bar as a result of erosion of the graphite block. Although erosion of carbon cathodes has also been observed, it is much less and does not impair the lifetime of the pots, which become unusable for reasons other than erosion of the cathode.
By contrast, the wear of graphite cathodes is sufficiently rapid to become the prime cause of death of aluminium electrolysis pots at an age that might be termed premature compared with the lifetimes recorded in the case of pots equipped with graphitic cathodes. Thus, the following wear rates of the various materials have been recorded:
FIG. 1 of the appended schematic drawing shows a cathode block 3, with the cathode current-supply bars 2, the initial profile of which is denoted by the reference 4. The erosion profile 5, depicted in dotted lines, shows that this erosion is accentuated at the ends of the cathode block.
The erosion rate of a graphite cathode block is consequently its weak point and its economic attraction in terms of increased production may disappear if the lifetime cannot be increased.
Although starting from different raw materials, carbon cathodes and graphite cathodes consist, in the end-product, of solid graphite grains and essentially differ in terms of the heat treatment given to the binder. The pitch of the graphitic product is treated, during firing of the product, at a temperature close to 1200xc2x0 C. The binder of the graphite cathode is heated, during graphitization, to a temperature above 2400xc2x0 C. and is therefore transformed into graphite.
The porosity of carbon and graphite cathodes results from the coking of the binder. However, this porosity is invaded, during operation of the pots by the electrolysis products, mainly sodium and aluminium fluorides. These products are therefore in contact with the carbon or the graphite coming from the binder.
The document Chemical Abstract Vol. 73, No. 22 teaches the impregnation of cathodes in order to block the pores and prevent the penetration of reactive products. This impregnation is done with products other than pitch and tar which, according to the author, are not effective as they do not wet the carbon enough.
The document JP 02 283 677 relates to electrodes for electrical discharge machining. The electrodes are impregnated and annealed before undergoing a graphitization heat treatment at 2600-3000xc2x0 C.
The document EP 0 562 591 relates to a method of impregnating carbon and graphite blocks at room temperature, using pitches treated with resins in order to obtain impregnation yields of greater than 40% after the impregnant has been carbonized. This document pertains neither to the electrolysis of aluminium nor the problem of the erosion of graphite cathodes.
The document JP 54 027 313 relates to an electrode impregnated with resins, for the production of chlorine.
The object of the invention is to provide a graphite cathode whose lifetime is increased. For this purpose, this cathode contains, within the pores of its structure, a carbon-containing product fired at a temperature of less than 1600xc2x0 C., improving the erosion resistance by protecting the graphitized binder.
The carbon-containing product is introduced by impregnating it into a graphite cathode obtained in a known manner.
The carbon-containing product fired at a temperature of less than 1600xc2x0 C. ensures, within the pores in the cathode, that the graphitized binder is protected and improves the erosion resistance of the cathode. This product is deposited on the graphitized binder, lining the pores, without blocking the pores which are necessary for the flow of products coming from the electrolysis bath. By being interposed between the products from the bath and the graphite binder, the impregnation product prevents the latter from being degraded by the reaction with the components from the bath which migrate into the pores of the cathode. Owing to its heat treatment at low temperature, compared with a graphite, the impregnation product is more resistant to attack by the components from the bath.
The carbon-containing product protecting the graphitized binder is chosen from coal pitches and petroleum pitches.
According to one method of implementation, the process for obtaining such a cathode consists in injecting the carbon-containing product in liquid form into the pores, protecting the graphitized binder. By way of example, if the carbon-containing impregnation product is a coal pitch, this is heated to a temperature of about 200xc2x0 C. in order to obtain a satisfactory viscosity.
One process for producing the cathode according to the invention consists firstly, in a manner known per se, in producing a cathode from coke, with or without graphite, and from pitch subjected to a heat treatment at a temperature greater than 2400xc2x0 C., in placing this cathode in an autoclave after optionally preheating it to a temperature corresponding to the temperature at which the impregnation product has the desired viscosity, in creating a vacuum in the autoclave, in introducing the impregnation product in liquid form into the autoclave, until the cathode is completely immersed, in breaking the vacuum in the autoclave by injecting a pressurized gas in order to allow, depending on the duration of the treatment, partial or complete filling of the pores in the cathode with the impregnation product, in returning the autoclave to atmospheric pressure, in removing the cathode from the autoclave and, finally, after possible cooling, in carrying out a heat treatment at a temperature of less than 1600xc2x0 C., but sufficient for the impregnation product to undergo curing and/or coking, thus forming a non-graphitized carbon layer which protects the graphitized binder from erosion.
The purpose of the heat treatment carried out after impregnation is to stabilize the impregnation product. This may be necessary in specialized potlines or during preheating of the electrolysis pot and during operation of the latter.
It may be noted that the impregnation may be carried out over the entire cathode, or only over part of it. When only partial impregnation is desired, it is necessary to render impermeable the surface of the block to be treated, or else to immerse the block only partially in the impregnation liquid.
In order to enhance the action of the treatment, it is possible to carry out, if so desired, several successive impregnation and firing cycles.